Chaya Sar, also known as Athar, was an Ascended Lantean sentenced to protect the planet Proculus in the Pegasus galaxy, after she disobeyed orders from the Others to remain out of corporeal affairs when she acted to protect her former home planet from a Wraith attack. As punishment, she was permitted to protect only her people, unable to aid any others in the Pegasus galaxy even as she knew what they were going through.
When she destroyed two Wraith Darts in pursuit of Major John Sheppard's Puddle Jumper in 2004, the Jumper landed and the team eventually made contact with her, with Dr. Rodney McKay assuming that the planet's protection was some kind of advanced weapon. Attempting to ask Chaya, who posed as her own high priestess, to permit her planet to become a 'refugee' world for other peoples whose homes had been destroyed by the Wraith, the team took her back to Atlantis.
While there, she developed a certain relationship with Sheppard, but McKay became increasingly suspicious of her when she was revealed to be in strangely perfect health despite her seemingly primitive background, as well as when she was shown to possess the Ancient Technology Activation gene by activating the biometric sensor array.
After McKay deduced that she was an Ancient, Chaya admitted the truth and, detecting a Wraith attack on her home world, she returned to it through the Stargate followed by Sheppard in a Puddle Jumper. After Sheppard attacked the fleet, Chaya appeared to him and told him to let her handle it, which she did. Using her powers, Athar destroyed the fleet made up of two Wraith cruisers and hundreds of Darts. Landing on the planet, Athar explained her history to Sheppard, and regretfully told him that the Others would not permit her to help any new arrivals on the planet. Despite this, before John departed, she 'shared' herself with him, allowing the two of them to witness everything that the other was. (SGA: "Sanctuary")